The present invention relates broadly to a voltage controlled oscillator apparatus, and in particular to a low phase noise two port voltage controlled oscillator apparatus.
The state of the art of voltage controlled oscillators is well represented and alleviated to some degree by the prior art apparatus and approaches which are contained in the following U.S. Patents:
U.S. Pat. No. 4,075,580 issued to Sunkler on Feb. 21, 1978;
U.S. Pat. No. 4,246,550 issued to Cohen on Jan. 20, 1981;
U.S. Pat. No. 4,310,809 issued to Buck et al on Jan. 12, 1982;
U.S. Pat. No. 4,450,416 issued to Mears on May 22, 1984;
U.S. Pat. No. 4,481,487 issued to Brehm et al on Nov. 6, 1984;
U.S. Pat. No. 4,484,157 issued to Helle et al on Nov. 20, 1984
Buck et al disclose a low noise voltage controlled oscillator which is based on a grounded collector junction transistor which includes both a coarse tuning voltage port and a fine tuning voltage port which are coupled respectively to the base terminal. The coarse tuning and the fine tuning circuits both utilize tuning varactors. Helle et al illustrates the operation of a wide frequency range voltage controlled oscillator in which the output is taken from the emitter of a junction transistor. In Mears the output is taken from the source of a FET. The Brehm et al discusses the use of wide tuning range varactors. The Cohen utilizes the wide tuning range features of lumped circuits to provide a wideband, millimeter wave frequency Gunn oscillator apparatus. The Sunkler utilizes the series connection of a voltage variable capacitor and a line element to provide a wide band junction transistor oscillator. However, the problems of phase noise generation by high Q varactors, such as gallium arsenide varactors is not addressed. Wherein the present apparatus utilizes large silicon varactors to suppress phase noise generation in a wide band voltage controlled junction transistor oscillator.
It is common in modern broadcast stations, to use crystals to maintain the frequency accurately to 2 or 3 cycles. Where a continuously adjustable frequency output is needed, some type of self-controlled oscillator is needed. The Hartley oscillator is well known in the art and until very recently extensively utilized, since it is one of the simplest. Its operation depends upon the energy which is fed back from the plate to the grid circuit through the inductive coupling of the two sections of the coil. The frequency is determined by the inductance and capacitance values in the tuned circuit.
The Colpitts oscillator which is also very well known is similar circuit. It differs from the Hartley circuit only in the manner in which energy is fed back from the plate circuit to the grid circuit, the coupling being accomplished by a capacitive voltage divider rather than a tapped coil.
The Colpitts oscillator is still in use today and has been updated to use the current semiconductor technology. In generation, a parallel combination of two capacitors and an inductor form a variable inductor which series resonates with a third capacitive reactance to form a conventional transistor Colpitts oscillator in which the transistor intrinsic capacitances--C.sub.BE & C.sub.CE --are used to generate the negative resistance. However, this circuit has been found to be incapable of achieving the desired noise performance over a 15% band as is frequently required.
Another prior art technique provides a voltage controlled oscillator in which fine tuning and coarse tuning are achieved by tuning a single varactor, or back to back varactors, through the summation of coarse tune voltage and fine tune voltage for special video circuits. The noise performance was found to be relatively good, but did not meet the requirement of 150 db/Hz at 1 Mhz. The tuning port modulation bandwidth of this type of circuit considerably less than 50 Mhz.